Sound levels and their relevance

Decibels are measured on a non-linear or logarithmic scale. This means that instead of sound intensity increasing in equal increments, each sound interval increases by a factor of the base of the logarithm.

For noise, this means that a small change in the number of decibels can result in a significant change in the intensity of noise and hence its potential to damage a person’s hearing.

The 3dB ‘trading effect’ means for every 3 dB the sound level increases the impact on hearing health which is doubled. For example, 63 dB(A) is twice as noisy as 60 dB(A). The relevance of this is apparent when considering exposure to hazardous level of noise. For every 3dB the noise level increases, the exposure time must be halved to keep the worker safe from harm.

The 3 dB trading effect is accepted in most countries. Some countries like the USA use a 4 dB and 5dB trading rule.

Examples of sound/noise levels generated in a range of activities:

• normal speech is around 60 dB(A)
• HGV passing close by is around 90 dB(A)
• a gunshot is around 110 dB(A).

Noise exposure limits

Globally occupational safety and health legislation requires organisations to provide workers with a safe and healthy workplace by controlling exposures to occupational hazards. Noise is a well-recognised occupational hazard which needs to be controlled.

This is achieved through the implementation of established occupational exposure limits for noise in the workplace. As countries set their own exposure limits for noise, practitioners will need to consult local legislation to determine what levels have been set for the country in which their organisation operates.

Although the limits vary from country to country, there is a generally accepted standard of a time weighted average (TWA) of 85 dB (A). TWA is a method of calculating a workers’ daily exposure to hazards such as noise, it refers to the average rate at which a worker is exposed to an adverse condition such as noise without unpleasant or dire effects over a defined period such as an 8hour day or 40 hour work week.

The A-weighting is applied to instrument-measured sound levels (usually using a sound meter device) to account for the relative loudness perceived by the human ear, as the ear is less sensitive to low frequencies. Put simply- A weighting measurements are designed to mimic how humans intuitively perceive noise that is below 100 decibels in volume.

The loudness of the noise is measured in A-weighted decibels and is abbreviated to dBA. The dBA scale closely matches the loudness of sounds as perceived by the human ear. What this means is that when measuring the frequencies and decibels of sound with a sound level meter, the frequencies in the 500 to 10 000 Hz range are measured filtering the lower frequencies, which are not usually heard by the human ear. A c-weighted range on the other hand would measure frequencies 30 to 10 000 Hz.

The importance of the TWA is that the longer the duration of exposure the greater the risk of harm. When a worker is exposed to noise level below the established exposure limit for a maximum of 8 hours in a day there should be no negative impact on hearing. When workers are exposed to noise levels louder than the established exposure limits then the duration of exposure must be reduced to protect the workers hearing.

As well as TWA, many countries also implement an exposure limit value for peak sound pressure. This peak sound pressure is a limit that workers should not be exposed to and is not weighted by time.

Directive 2003/10/ECs a legal act provided for in the EU, has set limits for noise exposure at work. The Directive came into force in 2006 and requires organisations to measure the levels of noise to which workers are exposed (if necessary). It also states minimum requirements for the protection of workers from risks to their health and safety which are likely to occur from exposure to noise.

The Directive recommends three action levels for occupational settings depending on equivalent noise levels for an average 8 hour working day:

(a) exposure limit values: LEX,8h = 87 dB(A) and ppeak = 200 Pa ( 1 ) respectively

(b) upper exposure action values: LEX,8h 85 dB(A) and ppeak = 140 Pa ( 2 ) respectively

(c) lower exposure action values: LEX,8h = 80 dB(A) and ppeak = 112 Pa ( 3 ) respectively.

What is the difference between exposure, upper exposure, and lower action values?

The ‘action level’ is a noise exposure level at which if met, the organisation is required to take action to reduce the level, however, the exposure limit value of 87 dB(A) which no worker can be exposed to.

The different levels mean different controls, the lower levels may require training and hearing protection whereas, the upper levels may require additional controls to reduce the level or the length of exposure time. These will be decided through carrying out a risk assessment, ensuring the noise limits have been assessed to determine the controls required.